Optical refrigeration can include laser excitation of rare-earth doped glass and crystal host material systems. In some systems, optical refrigeration can include use of ytterbium doped fluorozirconate glass (Yb:ZBLAN) or ytterbium doped yttrium lithium fluoride crystal (Yb:YLF). Potentially, a high purity semiconductor crystal (double heterostructure or multiple quantum wells) could also be used as the cooling material.
In an optical refrigeration system, a cooling cycle is based on conversion of low-entropy low-energy input photons of an optical field (e.g. laser) into an isotropic higher-energy spontaneous emission (fluorescence) photons. The excitation laser is red-shifted from a mean wavelength of the emitted fluorescence (λf). Following absorption, out of equilibrium excitation becomes thermalized within the ground and exited state manifolds of the rare-earth ion. This is accomplished by phonon absorption from lattice vibrations of a material host. Thermal quanta of energy kT are carried away from the host in a form of spontaneously emitted photons, thereby cooling the material.
Laser cooling of solids can be exploited to achieve an all-solid-state cryocooler 100 as conceptually depicted in FIG. 1. The cryocooler 100 will exhibit advantages of compactness, no vibrations, no moving parts or fluids, high reliability, and no need for cryogenic fluids. In general, the cryocooler 100 will include a housing 110 a cooling crystal 120 within the housing, high reflectivity mirrors 130 at opposing ends of the cooling crystal 120, and a laser source 140 directed at the crystal 120. The housing 110 confines the crystal 120 within a vacuum space 115, and a combination thermal link 150 and IR detector 160 enable transfer of the cooled crystal state and detection of the crystal temperature, respectively. A heat sink 170 surrounding or supporting the housing 110 can absorb heat released from the crystal 120. In FIG. 1, pump light is generated by the semiconductor diode laser 140 and carried to the mirrored cooler element 120 by an optical fiber. The laser enters the cooler element 120 through a pinhole 135 in one mirror 130 and is trapped by the mirrors 130 until it is absorbed. Isotropic fluorescence escapes the cooler element as shown by radiation lines 125 and is absorbed by the vacuum casing. A sensor or other load is connected in the shadow region of the second mirror
Space-borne infrared sensors can benefit from an all-solid-state cryocooler, as will other applications requiring compact cryocooling. In many potential applications, requirements on pump lasers are not very restrictive. The spectral width of the pump light has to be narrow compared to the thermal spread of the fluorescence. A multimode, fiber coupled laser with spectral widths of several nanometers would be sufficient. In an optical refrigerator, the cooling power is on the order of one percent of the pump laser power. For micro-cooling applications, with mW heat lift, only modest lasers are needed. For larger heat lifts, correspondingly more powerful lasers are needed. In all cooling applications, the cooling element has to be connected to the device being cooled, the load, by a thermal link. This link can siphon heat from the load while preventing the waste fluorescence from hitting the load and heating it.
It will be expected that any increase in cooling capacity of a crystal based cryocooler will require an increase in pump laser power, and a corresponding increase in size and complexity of, for example, the cryocooler 100 depicted in FIG. 1.
These challenges can be been dealt with herein, using a combination of optically-pumped semiconductor lasers (OPSL) together with an intracavity enhancement method to construct a compact optical cryocooler, as will be described in connection with the exemplary embodiments that follow.